TRANSPIRATION. 1 1 5 



some ground for the belief that it plays an important part in 

 the distribution of these salts throughout the plant ; and 

 again, we know from the observations of Hales and von 

 Hohnel that the gases in the vessels of an actively transpiring 

 plant are at a lower pressure than the air, a condition which 

 materially affects the movements of gases and of water in 

 plants. These points we will now discuss in detail, but before 

 doing so we must not omit to notice one very important 

 influence which transpiration exerts, the full consideration of 

 which will come in a subsequent lecture. Inasmuch as loss 

 of water by a plant affects the turgidity of its cells, and since 

 turgidity is a necessary condition of growth, it is clear that 

 the transpiration of a plant and its growth must stand in 

 some more or less direct relation to each other. 



We have already learned that transpiration promotes 

 absorption of water by the roots, although these two functions 

 are not proportional. It often happens that the loss of water 

 by transpiration is so great that it cannot be met by the 

 absorbent activity of the roots : in fact it appears to be the 

 rule that, under ordinary conditions, the amount of water 

 absorbed by the roots in a given time is less than that which 

 is transpired by the leaves. Thus Sachs and de Vries found 

 that the root-stock of a Sunflower exuded at its cut surface 

 much less water (2*1 c.c.) than was transpired by the upper 

 leaf-bearing part of the stem (9*5 c.c.) in the same time. 

 Transpiration must therefore draw upon the reserves of water 

 which the plant contains. When these are becoming exhausted, 

 the leaves, and if the plant is a herbaceous one the stem also, 

 begin to droop, for the parenchymatous cells have lost their 

 turgidity and have become flaccid : they may be restored to 

 their normal condition by arresting the transpiration, or, in 

 the case of a cut-off branch, by forcing water under pressure 

 into the cut surface. If the transpiration of a plant be 

 prevented, the absorption of water by the roots gradually 

 diminishes, and finally ceases when the plant has taken up as 

 much water as it can contain. 



It is, therefore clear that transpiration has a direct influence 

 upon the absorption of water. This naturally suggests the 



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